The fabrication of integrated circuits has encountered problems in maintaining submicron precision that must be maintained over a macroscopic distance for lithographic processes. In the past, lithographic processes have used electron beam scanning. Electron beam scanning controls an electron beam which serially scans over the surface of a silicon wafer to create the desired image. This process typically takes several hours for the single electron beam to scan the entire wafer during fabrication. Generally, excellent resolution flexibility is possible when using a single beam instrument; however, the sequential operation can only scan over a limited area. Thus, to scan the entire wafer, a step and repeat sequence must be used with subsidiary alignment markers being picked up in each frame. Throughout most of the fabrication sequence of a typical integrated circuit, a series of fixed and repetitive exposure patterns are required; hence, the process of aligning and serially scanning is time consuming and ineffective for high volume production.
A high resolution electron image projection tube has been investigated for fabricating large arrays of micron sized transistors. See T. W. O'Keefe, J. Vine and R. N. Handy, An Electron Imaging System for the Fabrication of Integrated Circuits, Solid-State Electron 12, 841 (1969). In an electron image projector, pattern details are transferred from a mask onto a silicon wafer using electrons as the information carrier. Electron image projectors operate by driving electrons from an air stable photocathode having a surface containing an image of the desired pattern. Subsequently, coaxial electromagnetic fields focus the electron image on the silicon wafer targets. In previously developed electron image projectors, ultra-violet radiation was used to excite electrons which were emitted from the mask to the wafer.
In its operation, electron image projectors are generally arranged as simple diodes with a wafer as the anode and an electron/emitting mask as the cathode. In the prior art, ultraviolet radiation has been used in order to excite the mask. Electrons are emitted through photo-enhanced emission; thus, the name Photocathode Electron Projector is used for these machines. Unfortunately, the cost and complexity of tools using this radiation source is excessive and burdensome.
Despite the advancements in electron image projectors, problems have occurred with the method of activating the electrons to be emitted on the silicon wafer from the mask. A need has consequently arisen for an electron image projector that is cost effective and that has improved quality control characteristics that will allow the projector to emit electrons with reliability and controllability.